1. Field of the Invention
The present invention relates to an electrophotographic photoconductor used in, for example, electrophotographic copiers, printers, facsimiles and complex machines thereof, an image forming apparatus; and a process cartridge.
2. Description of the Related Art
By virtue of their various advantageous properties, organic photoconductors (OPCs) have recently been used in a lot of copiers, facsimiles, laser printers and complex machines thereof, in place of inorganic photoconductors. The reason for this includes: optical characteristics such as wide light absorption wavelength range and large light absorption amount; electrical characteristics such as high sensitivity and stable chargeability; a wide range of materials usable; easiness in production; low cost; and non-toxicity.
However, organic photoconductors have a charge transport layer mainly containing, for example, a low-molecular-weight charge transporting compound and an inert polymer. Such organic photoconductors are generally soft and receive mechanical load derived from a developing system or a cleaning system after repetitively used for a long period of time in an electrophotogaphic process, causing a problem that they are degraded in mechanical durability such as abrasion resistance and scratch resistance. Several proposals to form a surface layer containing a three-dimensionally crosslinked polymer on organic photoconductors have been made in order to solve the problem about mechanical durability in organic photoconductors (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2000-066425, 2009-229549 and 2006-084711).
A first proposal is to form a surface layer containing a three-dimensionally crosslinked polymer formed through radical polymerization by applying UV rays or electron beams to a hole transporting compound containing two or more chain-polymerizable functional groups in the same molecule thereof (see, for example, JP-A No. 2000-066425). However, this proposal requires a large, complicated apparatus for applying UV rays or electron beams, which is problematic in productivity. With this proposal including applying UV rays or electron beams, the charge transporting compound is degraded, so that the photoconductor is problematically degraded in electrical characteristics.
A second proposal is to form a surface layer containing a three-dimensionally crosslinked polymer formed by curing a charge transporting compound having a reactive group such as a hydroxyl group (see, for example, JP-A No. 2009-229549). In this proposal, the reactive group such as a hydroxyl group contained in the charge transporting compound remains in the three-dimensionally crosslinked polymer as a highly polar component, causing degradation in chargeability and hence degradation in electrical characteristics of the photoconductor. In a high-temperature, high-humidity environment, the image density tends to decrease due to exposure to NOx gas generated from a charging device, indicating that the photocondutor is poor in environmental stability and stability to gas.
A third proposal is to form a surface layer containing a three-dimensionally crosslinked polymer formed by curing a reaction active species such as melamine and a charge transporting compound having a reactive group (e.g., a hydroxyl group) blocked with a protective group (see, for example, JP-A No. 2006-084711). This proposal can prevent the reactive group such as a hydroxyl group from remaining, but reactivity is poor between the reaction active species and the reactive group (e.g., a hydroxyl group) blocked with a protective group, so that the formed photoconductor is poor in mechanical durability.
As described above, provision of a surface layer containing a three-dimensionally crosslinked polymer on an organic photoconductor can improve the mechanical durability of the electrophotographic photoconductor to some extent, but the electrophotographic photoconductor does not satisfy the required properties. Then, several proposals to form a surface layer containing a three-dimensionally crosslinked polymer and a filler on organic photoconductors have been made in order to further improve mechanical durability (see, for example, JP-A Nos. 2005-099688 and 2000-330313).
A fourth proposal is to disperse a filler in a crosslinked resin layer containing a three-dimensionally crosslinked polymer formed by curing a tri- or more functional radical polymerizable monomer having not having a charge transportable structure and a radical polymerizable monomer having a charge transportable structure (see, for example, JP-A No. 2005-099688). However, this proposal includes applying UV rays or electron beams as in JP-A No. 2000-066425 and is problematic in productivity and electrical characteristics.
A fifth proposal is to incorporate, into a surface layer of a photoconductor, fine particles and a binder resin crosslinked with a blocked isocyanate formed by block-polymerizing a hydroxyl or carboxyl group-containing charge transporting compound (see, for example, JP-A No. 2000-330313). However, in this proposal, unreacted highly polar groups problematically degrade chargeability. In a high-temperature, high-humidity environment, the image density tends to decrease, indicating that the photocondutor is poor in environmental stability. In addition, the stability to gas is problematically degraded due to exposure to NOx gas generated from a charging device.
The third proposal describes that a filler is incorporated for improving mechanical durability. The three-dimensionally crosslinked polymer is degraded in mechanical strength and hence degraded in ability to retain the filler, resulting in that the obtained photoconductor does not satisfy satisfactory mechanical durability (abrasion resistance). The filler incorporated into the surface layer becomes a charge trap, potentially causing an increase in residual potential. Several proposals to reduce adverse effects caused by incorporating the filler into the surface layer have been made (see, for example, Japanese Patent (JP-B) No. 3802787 and JP-A Nos. 2004-233756 and 2007-233425).
A sixth proposal is to add an organic compound having an acid value of 30 mgKOH/g to 400 mgKOH/g to a protective layer formed of a thermoplastic resin and a filler, in order to prevent an increase in residual potential caused by the filler (see, for example, JP-B No. 3802787). However, in this proposal, an acid compound is easily attached to the protective layer in the presence of NOx or ozone gas due to the structure of the organic compound having a specific acid value. The obtained photoconductor generates image blur and may be degraded in stability to gas. Addition of an antioxidant can prevent the image blur. However, the antioxidant added allows the photoconductor be increased in residual potential and hence degraded in electrical characteristics.
A seventh proposal is a method using a polycarbonate resin having a specific structure in a protective layer of a photoconductor in order to reduce image blur caused by an organic compound having a specific acid value (see, for example, JP-A No. 2004-233756). However, the required abrasion resistance is difficult to obtain with the polycarbonate resin used in this proposal and the obtained photoconductor is problematically degraded in mechanical durability.
An eighth proposal is to add an amino compound having a specific structure to the uppermost surface layer of a photoconductor in order to reduce image blur caused by an organic compound having a specific acid value (see, for example, JP-A No. 2007-233425). However, in this proposal, the amino compound used not only degrades the mechanical durability of a photoconductor but also causes phase separation, resulting in that the obtained photoconductor is degraded in electrical characteristics. These techniques are suitable for a protective layer containing a filler in a thermoplastic resin and are not simply applied to a protective layer containing a filler in a cured binder. Moreover, a thermoplastic resin allows the photoconductor not to have satisfactory mechanical durability.
Therefore, even these proposals have not yet provided an electrophotographic photoconductor being excellent in mechanical durability, electrical characteristics, environmental stability and stability to gas and providing truly long service life and stable image quality. At present, there is a strong need to develop such an electrophotographic photoconductor